The requirements for monitoring and controlling stack gas pollutants have resulted in the development of solid electrolyte gas sensors having electrolyte compositions uniquely responsive to gaseous pollutants such as SO.sub.2, CO.sub.2, NO.sub.2, HCl and Cl.sub.2 U.S. Pat. No. 4,377,460 (Hirayama and Lin) relates to SO.sub.2, NO.sub.2, and CO.sub.2 gas sensor apparatus utilizing K.sub.2 SO.sub.4 or Na.sub.2 SO.sub.4 solid electrolyte for SO.sub.2 gas detection, NaNO.sub.3 solid electrolyte for NO.sub.2 gas detection, and Na.sub.2 CO.sub.3 solid electrolyte for CO.sub.2 gas detection. A mullite (3Al.sub.2 O.sub.3 .multidot.2SiO.sub.2) membrane containing Na or K impurities is utilized between the electrolyte portions of two separated cells. U.S. Pat. No. 4,427,525 (Lin and Hirayama) utilized the same electrolyte materials, but in a dual solid electrolyte cell configuration consisting of two cells, to provide signals indicative of both O.sub.2 and either SO.sub.2, NO.sub.2 or CO.sub.2. A doped ZrO.sub.2 membrane containing Na or K impurities is utilized, so as to support alkali cation conductivity as well as O.sup.= ion conductivity.
The above-referenced sensors are electrochemical concentration cells which sense the equilibrium of a gas species of interest and generate an EMF signal corresponding to the difference in partial pressure of the gas species across the solid electrolyte sensor. Typically, the solid state sensor includes an ion conductive solid electrolyte with electrodes disposed on its opposite surfaces. The stack gas, or monitored gas environment, contacts a sensing electrode while the opposite electrode serves as a reference electrode.
Conventional solid electrolyte compositions require operating temperatures of between about 600.degree. C. and 900.degree. C. to exhibit the desired ion conductivity to generate a suitable EMF signal. The accuracy of the EMF measurement depends in part on the effective sealing, or isolation, of the reference electrode from the monitored gas environment contacting the sensing electrode of the electrochemical cell. In a different design, U.S. Pat. No. 4,492,614 (Welsh) describes a concentration cell to detect HCl or Cl.sub.2 gas, where the solid electrolyte is selected from a group consisting of gas-impermeable SnCl.sub.2, BaCl.sub.2 or PbCl.sub.2. These materials can be doped to improve ion conductivity by addition of KCl. The solid electrolyte separates a sample gas chamber and a reference gas chamber. Instead of using a constant source of Cl.sub.2 reference gas, a solid reference material can be used which exhibits a constant chlorine activity, such as chlorides of aluminum, lithium, copper or sodium. This solid material would be contained in the reference gas chamber.
Hydrogen chloride (HCl) gas is generated during the combustion of chloride containing plastic materials, such as polyvinylchloride, polychloroprene, chlorinated rubber, and others in municipal and industrial incinerators. Incinerators, therefore, may need to be equipped with scrubbers to remove acidic gases as well as particulates on which HCl may be adsorbed. Hydrogen chloride is a very reactive gas and, once airborne, dissipates rapidly by secondary reactions in the atmosphere. Residual HCl gas in incinerator effluents should, therefore, be determined close to the source, preferably in the stack of the incinerator. What is needed is a sensor for HCl, which will allow elimination of expensive CaO or Y.sub.2 O.sub.3 doped ZrO.sub.2 membranes. It is a main object of the invention to provide such a sensor.